Periodic density functional investigation of Lewis acid sites in zeolites: relative strength order as revealed from NH3 adsorption

Different types of cationic species acting as Lewis acid sites as well as Na-counter ion in dealuminated zeolites are investigated, for the first time, using periodic density functional method. The energy of ammonia adsorption is taken as a measure for the acidity. The results reveal that the relative strength of the investigated active sites increases in the order: Na+ < Al(OH)2+ < AlO+ < Si+. Moreover, the interaction of NH3 with tri-coordinated aluminum dislodged in the framework is less favorable than that with the neighboring silicon ion.